Apparatus for the development of electrostatic images



O 6, 1964 R. M. SCHAFFERT APPARATUS FOR THE DEVELOPMENT OF ELECTROSTATIC IMAGES Filed D80. 19, 1960 2 Sheets-Sheet 1 INVENTOR ROLAND M. SCHAFFERT ATTORNEY Oct. 6, 1964 R. M. SCHAFFERT APPARATUS FOR THE DEVELOPMENT OF ELECTROSTATIC IMAGES 2 Sheets-Sheet 2 Filed Dec. 19, 1960 FIG. 3

United States Patent 3,152,012 APPARATUS FGR THE DEVELOPMENT (BF ELEQTROSTATIC IMAGES Roland M. Schatfert, Saratoga, Calif., assignor to International Business Machines Corporation, New York,

N.Y., a corporation of New Yorlr Filed Dec. 19, 1960, Ser. No. 76,948 6 Claims. (Cl. 118-4537) This invention relates to electrostatic printing and more particularly to an improved method and apparatus for developing electrostatic images.

As is Well known to persons skilled in the art, electrostatic printing involves the production of electrostatic images on the surface of a dielectric member or the like, the application of charged minute electroscopic pigmented toner particles to the image-bearing surface, and the fixing of the developed image before or after transfer to a print medium, such as paper. Various techniques have been devised for applying the toner particles to the image-bearing surface. Among these is the technique sometimes called impression development. The present invention particularly relates to an improved impression development technique and device.

In impression development, the surface of a carrier member, such as a sheet or roller, is coated or impregnated with a quantity of minute electroscopic pigmented toner powder particles, i.e., toner, to form an image-developing surface. The developing surface is then placed against the surface of the electrostatic imagebearing member. The placing of the developing surface on the image-bearing surface is performed so that virtually no relative motion occurs during the contact between the developer and image-bearing surfaces. Thus, there is substantially no wiping, patting, or other motion during contact as is usually found in other techniques (e.g. brush or cascade development) for applying the toner to the image-bearing surface. For the purpose of defining the present invention, the term impression and its derivatives shall denote as well as connote contact without relative motion during contact.

Heretofore, apparatus for impression development of electrostatic images has not produced print copy of favorable quality. This is particularly so in high-speed developing for high-speed continuous printing. The principal defect in such copy has been that the developed image or portions thereof usually are not uniformly dense throughout, that is, portions of the developed image have fewer particles than is required to produce clean sharp images for straight line copy. In continuous tone, the irregularities exist where there are excess quantities as well as insufficient quantities of particles in various portions of the image. Another defect has been spurious deposition of powder in background areas. Still another defect has been the lack of sufficient pow der (toner) provided by the developer surface to pro duce a dense deposition on the image, resulting in light prints.

These undesirable results have been found to be attributable to the lack of uniformity in the distribution of electroscopic particles forming the development surface of the impression member, lack of sufficient triboelectric adherance of the particles to the impression sur face, and lack of sufiicient capacity of the impression surface for holding a dense supply of particles. Previous methods for applying toner to form the development surface have involved cascading and/or brushing of particles as well as spraying onto the surface of the development member. However, these methods generally produce unevenness in the distribution of particles forming the development surface and do not provide sufiicient overall density for good prints. Consequently non-uniform and/ or undesirably light prints are obtained.

It is a general object of this invention to provide a new and improved impression developer method and apparatus for developing latent electrostatic images.

It is also an object of this invention to provide a new and improved impression developer method and apparatus whereby prints of im roved quality may be produced.

It is a more specific object of this invention to provide an improved im ression developer method and apparatus whereby electroscopic toner particles are applied in a more uniformly distributed manner to a surface bearing an electrostatic image.

It is a further object to provide an impression developer surface with higher capacity for electroscopic powder, providing denser prints.

It is a further object of the present invention to provide an improved method and device for impression development on a continuous and/or high-speed basis such as is required in electrostatic printing apparatus.

These and other objects are attained in accordance with the practice of this invention by applying electroscopic particles to the surface of a development memher in a manner which uniformly electrostatically saturates its surface. This is accomplished in accordance with the practice of this invention by first forming a transfer surface of electroscopic particles retainable on a transfer member surface and then impressing against the transfer surface, a uniformly electrostatically saturable surface of a development member having an electrostatic afiinity within its range of saturation, capable of overcoming the retention of particles by the transfer member. Thus a development surface is formed which contains a uniformly distributed quantity of toner particles which is then impressed onto an image-bearing surface.

In the preferred embodiment, the transfer member comprises a pile-like fabric material having a specific surface area much greater than unity and which is triboelectrically related to the electroscopic particles so that there is a mutually opposite electrostatic charging of particles and transfer member on contact. Thus the particles are contained in the fabric material and in its outermost interstices essentially through electrostatic forces of attraction.

Also in the preferred embodiment, the development member comprises a pile-like fabric material having a specific surface area much greater than unity and which is triboelectrically related to said particles so that a mutually opposite electrical charging of the development member and particles occur on contact. To effect the transfer of particles from the transfer surface to the development member on impression, the development member fabric is triboelectrically related to the particles and transfer member fabric such that a superior electrostatic force or affinity is exerted on the particles by the development fabric up to the point of its electrostatic saturation.

t) Thus, when the development fabric is saturated anywhere over its surface, the electrical force on the particles exerted by the transfer member prevents additional particles from being transferred to the development member fabric; consequently, uniformity is attainable with a uniformly saturable fabric. It has been discovered that particu larly desirable results are obtained with a transfer member comprised of a mohair fabric having a pile thickness of about /s and a development surface of rayon velvet fabric having a pile thickness of or less, where resin type electroscopic particles were employed.

Thus, it is apparent that by saturating the impression member, a uniformity of distribution of electroscopic particles is achievable. By impressing the saturable impression member on the layer formed on the transfer member, a uniformity of distribution is obtained on the impression member which has not been previously obtainable. One of the principal advantages realized from this invention is that the uniform saturation is obtainable when contact of the impression surface is made on a transfer surface which itself is not necessarily uniform provided every part of the transfer surface contains a supply of particles exceeding the predetermined saturable quantity. This advantage is particularly significant in that it permits the feeding and distribution of the developer particles to be performed more easily and with simpler techniques particularly where high-speed development is to be performed for printing on a continuous basis.

Another advantage is that continuous tone and halftone image, as well as line prints, can be developed rapidly and continuously and particularly where development is desired directly on paper overlaying an image-bearing surface.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 shows a printer apparatus incorporating the features of the present invention.

FIG. 2 shows a modification of a printer apparatus similar to that of FIG. 1, also incorporating the features of the invention.

FIG. 3 shows a further embodiment of a developing device incorporating the features of the present invention.

Referring now to the drawings, FIG. 1 shows an electrostatic printer apparatus of the type commonly referred to as a xerographic printer which is suitable for printing at high speeds on a continuous basis. A cylindrical support it) is mounted on an axle ll which is connectable to a constant speed drive mechanism of any suitable type. The outer surface of the support it) is coated with a photoconductive insulating surface 12 adapted to receive and retain electrostatic charge in the absence of light and to dissipate the charge upon exposure to light.

Electrostatic charge is produced on surface 12 at a charging station 13 comprising, for example, a positive polarity corona discharge electrode consisting of one or more fine wires suitably connected to a high voltage source 14 having a potential high enough to cause a corona discharge surrounding the wires which are located proximate surface 12. Subsequent to the charging station in the direction of rotation of the cylinder 19, is an exposure station 15 generally comprising means for imposing a pattern of light and shadow on the surface 12 and including, for example, a projection lens 16 or other exposure mechanism as is conventional in the art.

Next subsequent to the exposure station, is a developing station generally designated as 17 which will be further described hereinafter. eyond the developing station 17 is a transfer station adapted to transfer xerographic images 18 from the surface 12 to a transfer web 19 which passes from a feed roll 20 through guide rolls 21 into contact with cylinder surface at a point beneath a transfer roller Zii to and desirably subsequently through a fixing device 23 to a take-up roll 24. Fixing device 23 which may be a mechanism for applying heat to fuse xerographic images 18 to transfer web 19 is designed to fix the transfer image to the web to yield a permanent completed xerographic print.

Optionally a cleaning station 25 may be disposed adjacent to the surface of cylinder it along with other auxiliary devices or stations whereby a residual powder image electrostatic charge or other material or phenomenon may be cleaned or erased from surface 12. Thus, for example, a rotating brush 26 driven by a motor or the like (not shown) may be brought to bear against surface 12 to erase residual powder images therefrom.

The development station 17 comprises an impression roller 33 and a transfer roller 31 located within a housing 32 which may include an open-ended hopper 33 or the like above the transfer roller 31. A metering device comprises a rotatable cylinder 34 supported on an axle 35 within an opening in the plate 36 which forms the bottom of hopper 33. A supply of electroscopic particles 37 in hopper 33 by gravity feed contacts the peripheral surface 38 of cylinder 34 as it moves into the hopper 33. Axial grooves 33 in peripheral surface 38 of cylinder 34 serve to take up predetermined quantities of particles 37. The opening defining edges of plate 36 in the bottom of hopper 33 preferably serve as doctor blades to remove particles from peripheral surface 38 allowing only quantities in the grooves 39 to be dispensed outside and below the hopper 33. A rotatable brush 4G is provided to sweep particles from the grooves 39 in a distributed ream 41 onto the surface of transfer roller 31.

In accordance with this invention, impression roller 31' has a peripheral surface having a surface area much greater than unity. In the preferred embodiment, such a surface is provided by a pile-like fabric material preferably in the form of a sleeve 42 or the like which covers the peripheral surface of a conductive cylinder 43. The attachment of sleeve 42 on the cylinder 43 is made so that slipping, wrinkling, or the like cant occur so that a continuous non-moving surface is provided. Preferably cylinder 43 is mounted on axle 44 so as to be electrically in contact therewith, but electrically insulated from ground so that a controllable voltage can be applied between cylinder 43 and cylinder 1%) by source 56 connected by leads and 57 to axles 44 and 11 respectively. Impression roller 30 is supported in any well-known manner so that fabric 42 is pressed against the image-bearing surface 12 of drum it). A drive means comprising an electric motor 60 is connected to a multiple output gear unit 61. Connections in any wellknown manner are further made from gear unit 61 to shaft 11 of drum 10 and shaft 44 of impression roller 36 so that relative motion between roller surface 42 and image-bearing surface 12 during contact is Zero. With cylinder 43 being conductive and preferably in electrical contact with axle 44, the application of potential or grounding may be obtained to, in effect, make cylinder 43 a development electrode. By making source 56 variable the potential applied to cylinder 43 may be varied to thereby afford a measure of control over image density, contrast and background.

Similarly to impression roller 30, transfer roller 31 comprises a conductive cylinder 45 having a fabric sleeve 46 around it outer peripheral surface. Similarly sleeve as is attached to cylinder 45 so that slipping or wrinkling do not occur. Axle 47 supports cylinder 45 in any well-known manner such that fabric 46 presses upon the fabric 42. Driving means is provided for rotating transfer roller 31 at a speed such that the relative linear velocity of sleeve 46 and sleeve 42 is zero. Conveniently, the driving means may be part of the drive system for drum 10 and impression roller 30 and conveniently could include a connection from gear unit 61 to shaft 47 of transfer roller 31. A cylindrical brush 48 is rotatably mounted on axle 49 to press the fibers of brush 48 against surface of fabric 46 on transfer roller 31. Means (not shown) may also be provided for reciprocating brush 48 axially to effect an axial brushing action simultaneously with the pressing action to assure universal contact of particles 41 applied to surface of fabric 46. Other means such as a doctor blade, comb, or the like may be used instead of brush 48 to effect the agitation of particles 41 on surface of sleeve 46.

While in the practice of the present invention it is essential that the contact between surface 12 and the surface of fabric 42, and between fabrics 42 and 46 be by impression only, the brush 48 need not be in impression contact but can be permitted to have slippage. One of the advantages of the present invention is that considerable latitude is permitted in the means and method for applying electroscopic particles to the surface of fabric 46. For a continuous development operation useful for continuous high-speed electrostatic printing from images on a drum surface 12, the preferred embodiment includes a continually operable metering device which applies powder to surface of fabric 46.

In accordance with the practice of the present invention, the surface of impression roller 36 is uniformly electrostatically saturated and then pressed against imagebearing surface 12. The saturation of fabric is effected by applying a distributed quantity of electroscopic particles over the surface of transfer roller 31, the quantity being equal to or exceeding the amount necessary to effect saturation of surface 42. It has been discovered that if the triboelectric relationships between particles 41 and surfaces 42 and 46 are properly balanced, particles 41 will transfer from surface 42 up to a predetermined quantity of particles while any excess quantities of particles will be retained on surface 46. To accomplish this result, particles 41 are triboelectrically related to surface 46 so that surface 46 and particles 41 are mutually oppositely electrostatically charged. Thus, particles on surface 46 are retained thereon by an electrostatic field created by the triboelectric charging. Similarly, the surface 42 and particles 41 are also triboelectrically related so that surface 42 and particles 41 are likewise mutually oppositely charged. However, to effect the transfer of particles 41 from surface 46 to surface 42, the surface 42 in accordance with this invention must exert a force sufficient to overcome the retentive capabilities of surface 46. This transfer of course requires that the transfer force must be greater than the electrostatic attraction that exists between particles 41 and surface 46 plus the mechanical forces of adhesion that may also exist to retain the particles on surface 46. Depending on the triboelectric and mechanical characteristics of the particles, the materials forming surfaces 42 and 46 may be relatively far apart or close together in the triboelectric series.

The saturated condition of the impression surface 42 is maintained by a unique application of triboelectric principles. The technique can best be explained by reference to triboelectric behavior. In dealing with the triboelectric properties of materials, it is usual to place these materials in a triboelectric series with positive polarity at the top and negative polarity at the bottom. Thus, any material placed in contact with another material which is below it in the triboelectric series will become positively charged and the material below it in the series will become negatively charged. Thus, if a positively charged electrostatic image is to be developed, an electroscopic powder is selected which is below the impression surface material in the triboelectric series.

The electroscopic particles will then become charged to a negative polarity by contact with the impression surface and will be deposited upon the positively charged image by electrical attraction.

In the present instance, in accordance with the practice of the present invention, the material of transfer surface 46 is selected so that it is triboelectrically positive with respect to the electroscopic particles 41 but is triboelectrically negative with respect to impression surface material 42, i.e., it is not as far above particles 41 in the triboelectric series as is the impression surface material 42. Thus, when the impression surface 42 is brought into contact with the transfer surface 46, particles 41 will be attracted from the transfer surface to the impression surface 42. The impression surface 42 will become saturated with particles 41 when one or more contacts are made.

In this condition, surface 42 is used to develop electrostatic images 9 by impressing surface 42 into contact with insulating surface 12. The various portions of the impression surface 42 which contact the electrically charged portions of the image surface 12 will become more or less depleted of particles 41 during development, whereas the portions of impression surface 42 which contact the uncharged areas of surface 12 will remain saturated with particles 41. However, when the impression surface 42 is subsequently brought into contact with transfer surface 46, the depleted portions will attract more particles 41 from the transfer surface 46 so that the entire surface 42 again becomes uniformly saturated with particles 41. In those areas of surface 42 Where the depletion of toner did not take place, a transfer of particles 41 from transfer surface 46 to transfer surface 42 does not occur. Consequently, uniform distribution of particles 41 on the impression surface 42 is obtained.

While various substances" may satisfy the saturation and triboelectric transfer forces involved in the practice of the present invention, as has been previously stated, the surfaces 42 and 46 are preferably made from pile-like fabric material. One feature of such fabric material resides in its surface characteristic, i.e., the specific surface area is much greater than unity (where a perfectly smooth surface is equated at unity). Thus, a surface is said to have surface area greater than unity when its total effective area due to irregularities in surface is greater than the total area of an equal size perfectly smooth surface. For example, smooth rubber or metal such as copper, steel may be said to have unity specific surface area whereas cotton, silk, rayon, mohair, or short pile fabrics made of the same due to interstitial irregularities and protruding fibers provide greater effective surface area for the same overall area. With a specific surface area much greater than unity, considerably more particles may be carried by the surface member thereby making possible greater density.

Various materials have been found to possess the desirable characteristics set forth supra. For example, surface 42 may be formed of a fabric such as velvet cloth, suede cloth, porous rubber, and soft finish paper; and surface 46 from a fabric such as felt, or pile mohair where particles are a resinous pigmented mixture of the types set forth in one or more of the following patents, 2,788,288; 2,753,308; 2,638,416; 2,618,552.

In a preferred embodiment, the fabric for 42 consisted of rayon velvet having a pile thickness of not more than fabric for surface 46 consisted of mohair of pile thickness of A2"; and particles 41 consisted of a resinous mixture of the type set forth in US. Patent No. 2,788,288 with particle size ranging from 2-20 microns. It was found that by vigorously rubbing a mohair surface with an excess quantity of particles 41 with a brush or the like to assure a distribution of quantity of particles 41 in excess of the quantity needed to saturate surface 42, the rayon velvet fabric impressed thereagainst one or more times produced a uniform distribution of particles for impression development of positively charged electrostatic images on an image-bearing surface.

In a further embodiment, the fabric for surface 42 consisted of cotton velvet having a pile thicknesses of A the fabric for surface 46 consisted of Wool felt;

3" and particles 4-1 consisted of a resinous mixture of the type set forth in US. Patent No. 2,753,308.

FIG. 2 shows an apparatus in which the development station is the same as in FIG. 1. However, in FIG. 2, the electrostatic images are developed directly on a print medium. For that reason, print web 19 is fed from supply roll 2%} over guide rolls 27 to take-up reel 24- after passing between impression roll 30 and drum surface 12. The present invention has proved particularly advantageous for such application. Electrostatic attraction forces between particles 47 and sleeve 42 as well as the absence of significant wiping action during contact permits improved printing quality directly on ordinary paper with very low backgrounding.

The method of the present invention when applied to the development of electrostatic images directly on paper 19 has been found to be particularly effective for developing halftones and continuous tone subjects. Furthermore, by developing directly on paper by the impression method, the electrostatic image is not destroyed or disturbed, therefore, multiple copies can be produced from one electrostatic image. As in the case of FIG. 1, the voltage 56 may be varied to effectuate a measure of control over image density, contrast and background.

FIG. 3 shows an alternate embodiment in which the impression member is in the form of a continuous belt St). Rollers SI and 52 are arranged to impress belt in image-bearing surface 12 of drum it A third roller 53 coacts with rollers 51 and 52 to advance belt at a speed equal to the linear velocity of drum surface 12 and impresses web St? on the surface of transfer roller 31. For this purpose one output of gear unit 61 may be connected to roller 51 through shaft 62. As in apparatus of FIGS. 1 and 2, belt 59 is preferably made of pile fabric material. However, since the belt is not everywhere backed by conductive elements during impression contact with surface, a development electrode 54 suitably connected by lead 55 to appropriate potential vis-a-vis drum 1% is provided intermediate rollers 5t? and 51. A belt with a flexible conductive base could be used, however.

While the practice of the method which constitutes the present invention is shown and described in connection with an automatic apparatus, it will be appreciated that the method can similarly be practiced manually, or manually and mechanically in part. For example, the transfer surface 46 which could be a mohair covered flat member as well as a cylinder on which resinous electroscopic particles such as one referred to in U.S. Patent No. 2,788,288 are spilled by hand or a hand or automatically operated dispenser. A brush, comb, doctor blade or the like may be used manually to distribute particles over the entire surface to assure maximum triboelectric charging thereof. The mohair covered transfer member may then be turned on edge and vibrated or tapped lightly to remove excessive uncharged particles or these may be removed as part of the rubbing or brushing action which effects maximum triboelectric charging of particles.

A manually operable impression member would preferably be a roller comprising a handle and a cylinder supported thereby and covered with a rayon velvet sleeve attached to the outer surface. The impression roller is then roll impressed on the particle-impregnated transfer member one or more times with a light manual pressure to effect the uniform saturation of the rayon velvet fabric. A Xerographic plate or the like which has previously been processed to have electrostatic images formed thereon is then impressed with the saturated rayon velvet surface. In those areas of the roller which contact charged areas of the surface of the image-bearing plate, particles transfer and adhere to the surface to render the electrostatic image visible. In those areas contacting the non-image portions of the image-bearing surface of the plate, particles are retained on impression roller surface.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. Apparatus for developing a visual image from a latent electrostatic image on the surface of an image carrying member by distributing electroscopic powder thereon in a density pattern corresponding to the charged distribution in said image comprising a first roller member having a developer surface,

said developer surface being uniformly electristatically saturable by a predetermined quantity of said electroscopic powder,

said first roller member being positioned with said developer surface bearing against said image bearing surface of said carrier,

a second roller member having a transfer surface capable of retaining over substantially the entire surface thereof by triboelectric attraction at least said predetermined quantity of electroscopic powder,

said second roller being positioned with said transfer surface thereof bearing against said developer surface,

said developer surface, said transfer surface and said electroscopic powder being differentially triboeletrically related whereby said powder transfers to said developer surface on contact therewith when said developer surface is electrostatically unsaturated and is retained on said transfer surface when said developer surface is electrostatically saturated,

means for distributing electroscopic powder over the entire area of said transfer surface in an amount of at least said predetermined quantity,

means for moving said image bearing member, said developer member and said transfer member at mutually interrelated speeds whereby the relative velocities of said image bearing surface, said transfer surface and said developer surface are zero during contact.

2. An apparatus in accordance with claim 1 in which said developer surface of said first roller member is formed of a first fabric material uniformly electrostatically saturable by said predetermined quantity of said electroscopic powder,

and said transfer surface of said second roller member is formed of a second fabric material,

said first fabric material having a triboelectric affinity for said powder up to the level of the electrostatic saturation thereof greater than the retentive force of said second fabric material on said powder,

said second fabric material having a triboe.ectric affinity for said electroscopic powder greater than said first fabric material when said first fabric is electrostatically saturated.

3. An apparatus in accordance with claim 2 in which said first and second fabric materials are triboelectrically separated from said electroscopic powder,

said first fabric material being closer to said electroscopic powder in the triboelectric series than said second fabric material.

4. In an electrostatic printer apparatus a device in accordance with claim 2 in which said first fabric material is a rayon velvet and said second fabric material is pile weave mohair and said electroscopic particles include a resin capable of being electrostatically charged on contact with said rayon and said mohair fabric materials.

5. In an electrostatic printer apparatus in accordance with claim 2 said first and second fabric materials being pile-like.

6. in an electrostatic printer apparatus a device in accordance with claim 2 in which said first fabric material is wool felt and said second fabric material is cotton References Cited in the file of this patent UNITED STATES PATENTS Wilson Oct. 7, 1930 Burke et a1. Apr. 6, 1954 Stockdale et a1. Apr. 29, 1958 10 Mayo July 21, 1959 Greaves Sept. 8, 1959 Hider Nov. 8, 1960 Westervelt et a1. Mar. 14, 1961 Reuter Apr. 11, 1961 Streich Oct. 10, 1961 Johnson et a1 Nov. 28, 1961 Gundlach Dec. 5, 1961 

1. APPARATUS FOR DEVELOPING A VISUAL IMAGE FROM A LATENT ELECTROSTATIC IMAGE ON THE SURFACE OF AN IMAGE CARRYING MEMBER BY DISTRIBUTING ELECTROSCOPIC POWDER THEREON IN A DENSITY PATTERN CORRESPONDING TO THE CHARGED DISTRIBUTION IN SAID IMAGE COMPRISING A FIRST ROLLER MEMBER HAVING A DEVELOPER SURFACE, SAID DEVELOPER SURFACE BEING UNIFORMLY ELECTRISTATICALLY SATURABLE BY A PREDETERMINED QUANTITY OF SAID ELECTROSCOPIC POWDER, SAID FIRST ROLLER MEMBER BEING POSITIONED WITH SAID DEVELOPER SURFACE BEARING AGAINST SAID IMAGE BEARING SURFACE OF SAID CARRIER, A SECOND ROLLER MEMBER HAVING A TRANSFER SURFACE CAPABLE OF RETAINING OVER SUBSTANTIALLY THE ENTIRE SURFACE THEREOF BY TRIBOELECTRIC ATTRACTION AT LEAST SAID PREDETERMINED QUANTITY OF ELECTROSCOPIC POWDER, SAID SECOND ROLLER BEING POSITIONED WITH SAID TRANSFER SURFACE THEREOF BEARING AGAINST SAID DEVELOPER SURFACE, SAID DEVELOPER SURFACE, SAID TRANSFER SURFACE AND SAID ELECTROSCOPIC POWDER BEING DIFFERENTIALLY TRIBOELETRICALLY RELATED WHEREBY SAID POWER TRANSFERS TO SAID DEVELOPER SURFACE ON CONTACT THEREWITH WHEN SAID DEVELOPER SURFACE IS ELECTROSTATICALLY UNSATURATED AND IS RETAINED ON SAID TRANSFER SURFACE WHEN SAID DEVELOPER SURFACE IS ELECTROSTATICALLY SATURATED, MEANS FOR DISTRIBUTING ELECTROSCOPIC POWDER OVER THE ENTIRE AREA OF SAID TRANSFER SURFACE IN AN AMOUNT OF AT LEST SAID PREDETERMINED QUANTITY, MEANS FOR MOVING SAID IMAGE BEARING MEMBER, SAID DEVELOPER MEMBER AND SAID TRANSFER MEMBER AT MUTUALLY INTERRELATED SPEEDS WHEREBY THE RELATIVE VELOCITIES OF SAID IMAGE BEARIGN SURFACE, SAID TRANSFER SURFACE AND SAID DEVELOPER SURFACE ARE ZERO DURING CONTACT. 